In a metal processing line such as a cold-rolling line and a continuous annealing line, a running steel strip (hereinafter also called a strip) is sometimes buckled in the width direction because of various reasons including an uneven stress distribution on the strip due to a shape of the strip or a shape of rolls and a projection called a strip width reduction (or a buckling) is generated.
If a strip width reduction is generated and the steel strip is kept running, the strip width reduction will grow and may lead to a break of the steel strip. Since a break of the steel strip in a metal processing line significantly reduces productivity, generation of a strip width reduction has to be found at an early stage.
As a method to find a strip width reduction at an early stage, there are a visual inspection method with an ITV and a method in which a parallel light such as a laser beam is emitted by a light source on one side of a roll and the light is received by a photo-receiver on the other side of the roll to detect a strip width reduction through a change in an amount of received light as described in Patent Documents 1 and 2.
However, in the visual inspection method with an ITV, a strip width reduction might not be found due to a human error or an unclear image and the method requires continuous monitoring. Further, the method in which a parallel light is emitted on one side of a roll described in Patent Documents 1 and 2, is affected by a flutter of the running steel strip, and therefore the apparatus can be installed only around a roll where a flutter of the steel strip hardly occurs. A flutter of the steel strip means a change of position in a direction vertical to the surface of the steel strip. Further, the method requires troublesome pre-adjustment including adjustment of positions at which the light source and the receiver are set and the method may be affected by environment such as temperature and dusts and therefore may lead to a midsection.
Thus, a projection (strip width reduction) detecting apparatus and a projection detecting method which enable a high-accuracy detection of a strip width reduction even with a flutter of the strip and under adverse surrounding environments in a metal strip processing line have not been developed. More generally, a projection detecting apparatus and a projection detecting method which ensure detection of a projection formed on surface of a running metal object continuously in the running direction even when a position in a direction different from the running direction of a running metal object changes and the surrounding environments are adverse ones have not been developed.